Making mistakes when writing sales copy isn't difficult, especially when you're first starting out. All you have to do to avoid making them, however, is recognize what those mistakes are. Before you get to creating actual copy, practice writing a few pieces so that you can get better. Here are a few of those mistakes so that you can avoid committing them in the future.
When you use a negative tone in your copy, it may turn your readers off and that's not very productive. That's why you'll always want to ensure you're speaking positively. A great way to handle this is to cross out all the negative adjectives and put words that make you feel good in their place. If you see that you've written something like stupid, replace it with something like smart. Negative words can work in your title to evoke anger or curiosity, but the bulk of your copy should use positive sounding words. Your copy's goal should be to effect people positively. The reader must get from you a good overall feeling about what they're buying, and about the entire buying experience. Your reader should come away feeling good, not bad.
Don't give too much away, or too little, as these are major mistakes that lots of copywriters make. That's often the most difficult part about writing copy, discovering just how much to give the prospect information-wise. Properly cutting the information back, or adding to it, is a very important step. You need to be able to give just the right quantity of information to the customer. So your job is to provide enough information to generate interest and get their attention, but at the same time keep it under control. The only way to determine how much you should offer the prospect is to develop the skills required. The reason your prospect is reading your sales letter is because they want more information about the product before they buy it. This should only include the benefits and the background of whatever you're offering, not anything technical about it.
Where you place the price of your product in your copy is critical. So don't make the mistake of letting the readers know the price of your product right in the start of the copy, this just kills the curiosity and besides that your prospect might not go through your benefits and just leave. You need to show your readers what you are offering them and tell them of the benefits before informing them of the cost of the product. You need to make sure that when you mention your price, it is quite clear. Your prospects need to see it clearly so avoid hiding it in your sales copy. You need to make sure it's clear and stands out, at the right time of course.
In closing, you need to work hard to make good copy, so you should try to avoid these common mistakes.